1. Field of Invention
This invention relates generally to an anchor which when installed in a hollow wall is adapted to receive a fastener screw to hold an object adjacent the wall, and more particularly to a dagger-like sheet metal that is drivable into the wall, thereby dispensing with the need to pre-drill a hole therein, the anchor when installed and the fastener screw is received therein, creating a truss within the body of the wall which acts as a reverse truss to resist pull-out forces.
2. Status of Prior Art
In order to mount fixtures, brackets and other objects adjacent the outer surface of a wall formed of plasterboard, sheetrock, fiberboard or other material employed in hollow wall construction, the conventional practice is to use an anchor for this purpose made of synthetic plastic material or metal. When wedged into a hole pre-drilled in the wall, the anchor is adapted to receive a fastener screw. This screw goes through a mounting hole in the object to be fastened to the wall so that when the fastener screw is fully received in the anchor, the object is then held in place adjacent the wall.
The installation of a conventional anchor of this type entails three distinct operations. The first and most critical step is to drill a hole in the wall of the right diameter. Should an oversized hole be drilled, then in the next step in which the anchor is pushed into the drilled hole, the anchor will not wedge firmly therein. Hence it would not be possible to carry out the third step which is to turn the fastener screw into the anchor, for if the anchor is loose within the hole, turning the screw will also rotate the anchor and the screw will not advance.
A householder or do-it-yourselfer, in order to install a conventional anchor, requires not only a manual or an electrially operated drill for this purpose, but also a drill bit of the proper diameter. The need to drill a hole in a hollow wall having a diameter appropriate to the size of the particular anchor to be installed, has discouraged many non-professionals from installing such anchors.
Recognizing the desirability of an anchor that does not require a drilling operation to install, self-drilling anchors have been developed to satisfy this need. One such self-drilling anchor is disclosed in the Giannuzzi patent U.S. Pat. No. 5,234,299 in which the anchor which is turned into a wall by a screwdriver, has an externally-threaded shank section whose head is engageable by the screwdriver, the shank section having a longitudinal bore to receive a fastener screw. Extending axially from the shank section is a core drill section which when the anchor is turned in by the screwdriver, bores a round hole in the wall which is then tapped by the turning shank section.
The holding power of an anchor is determined by the force it takes to pull the installed anchor out of the wall. This force is produced by the object fastened to the wall, and if the object is heavy, the holding power of the anchor may be inadequate to prevent the object from pulling it out of the wall.
In a conventional anchor of the hollow plug type that goes into a hole drilled in the wall, when the fastener screw is advanced through the cavity in the body of the plug, it acts to expand this body and in doing so to compress the wall material surrounding the hole, thereby wedging the anchor firmly within the hole. But since this material is usually plaster, or formed of fibers, or bonded particles lacking in structural strength and easily pulverized, the holding power of a conventional anchor is not high. However, it is adequate for those installations in which the object that is fastened to the wall is relatively light in weight.
The holding power of a self-drilling anchor in which an externally-threaded shank section taps the hole drilled in the wall by the drill section of the anchor does not depend on a wedging action, but on the fact that the internal thread tapped in the material of the wall surrounding the hole mates with the external thread on the shank section of the anchor. But since the internal thread is cut in plaster or other frangible or friable hollow wall material, the holding power of a self-drilling anchor is not high.
A wall anchor that is easy to install, yet has reasonably good holding power is the sword-like anchor disclosed in the Vassiliou patent U.S. Pat. No. 4,500,238 and in the Nowak et al. patents U.S. Pat. Nos. 4,765,788; 4,874,277 and 4,941,240, all of which disclose anchors of the Vassiliou type. These anchors are installed simply by driving them into a wall with a hammer, as one would a nail.
The Vassiliou anchor is fabricated of sheet metal blank that is cut, shaped, folded and heat treated to define a head having a center hole thereon, a flat neck depending from the head provided with an opening in the plane of the neck in alignment with the hole, and a pair of juxtaposed blades extending from the neck. When this anchor is driven into a hollow wall, the head of the anchor then lies against the outer surface of the wall, the neck is embedded in the body of the wall, and the blades then extend beyond the inner surface of the wall. And when a fastener screw enters the hole in the head and passes through the neck opening, as the screw continues to advance beyond the neck, it acts to spread apart the blades behind the wall.
The flat neck embedded in the wall material offers little resistance to pull out. The holding power of the Vassiliou anchor therefore depends mainly on the resistance to pull out produced by the spread-apart blades. This resistance is limited, for the blades are formed of spring metal, and when the anchor is subjected to a heavy pull-out force, the spread out blades can inwardly deflect thereby reducing their resistance to pull out through the wall material.
Inasmuch as an anchor in accordance with the invention, creates a truss when it is installed in a wall and a fastener screw is received therein, of background interest is the Giannuzzi patent U.S. Pat. No. 4,828,439 which discloses a plastic anchor, that when inserted in a hole drilled in a hollow wall and a fastener screw is inserted therein, creates behind the wall a triangular truss in which the screw forms a central strut.